The use of sounding-derived indices for a neural network short-term thunderstorm forecast

In this paper several indices derived from atmospheric sounding data are used to develop a short-term thunderstorm forecast tool. The Udine ( Italy) sounding ( WMO code 16044, taken every 6 h, hereafter 6 h) is used to describe the initial conditions in which a thunderstorm may develop in the Friuli Venezia Giulia region. The tool forecasts the convective activity in the 6 h after the sounding is launched. Sounding, lightning, and mesonet station data, from April to September of the years 1995 - 2002, were used to train and validate artificial neural networks ( ANNs) and the data of 2003 were used as an independent test sample. Special emphasis was given to avoid one of the major ANN problems, that of data overfitting, which requires limiting the possible complexity of the ANN, that is, the total number of inputs ( predictors) and of hidden neurons. To select the best input set, the training - validation mechanism was repeated in eight different ways. Two types of ANNs were developed: the first is a classification model and is built for forecasting the thunderstorm occurrence, defined as the report of at least one cloud-to-ground lightning strike. If this first ANN forecasts convective activity, then a second ANN, built as a regression model, is used for forecasting the thunderstorm intensity. The relative operating characteristic ( ROC) diagram and some parameters derived from a contingency table were used to compare the different classification performances. The linear cross-correlation coefficient, R-2, was used to assess the performance of the regression ANNs. The ANN inputs were not the only " raw" sounding-derived index values; in addition, two derived sets of variables, corresponding to the z scores and the " posterior probability" transformed data, were used as preprocessing methods. The last method yields the best results. The tool reported on here has been used operationally since 2001 in the regional meteorological office of the Friuli Venezia Giulia region [ Osservatorio Meteorologico Regionale/Agenzia Regionale per la Protezione dell'Ambiente ( OSMER/ARPA)].